1. Field of Invention
The present invention relates to Light Emitting Diode (LED) driving circuits. More particularly, the present invention relates to LED backlight driving circuits.
2. Description of Related Art
Electronic devices, such as mobile phones, laptop computers, and personal digital assistants (PDAs), frequently use liquid crystal displays (LCDs) or plasma displays as monitors. However, the LCD or plasma display panels themselves do not generate light. There should be a backlight module mounted to the back of an LCD or plasma panel to provide the light required by the panel. Currently used backlight modules include flat fluorescent lamps (FFLs), cold cathode fluorescent lamps (CCFLs), and light emitting diodes (LEDs). LEDs have become the most promising backlight sources not only because LEDs have small volumes, low heat radiation, low power consumption, long life, and short response time, but also because LEDs have overcome many problem that used to hamper the development of FFL and CCFL backlight modules.
An LED backlight module generally has a plurality of LED strings. Each of the LED strings is made up of one or a plurality of serially connected LEDs. A driving voltage with a constant voltage level is used to drive all the LED strings. In order to adjust the light luminance of each LEDs, the pulse width modulation (PWM) technique is used to adjust the duty cycle of the driving voltage. However, the inevitable variation in raw material and manufacturing process will cause the LED strings in a backlight module to have different conducting (i.e. turning-on) voltages. Because a driving voltage with a constant voltage level is used to drive the red-light LEDs, the green-light LEDs, and the blue-light LEDs, the three lights will have incoherent light luminance. The incoherent light luminance of the three lights will deteriorate the light quality of the resulting white light. One solution to the above problem is to utilize a constant driving voltage with a higher voltage level to drive all the LED strings. However, the solution will waste more power and curtail the life of the overdriven LEDs.